Welcome Janet! If Kaycee is diabetic, it is absolutely worth treating him! Diabetes is very manageable in cats, and you have come to the right place to learn how to do it. Even at 20 years old, it does not have to shorten his lifespan at all. I would suggest that you read the links that Hillary put in her post further up in this thread. Start with the ones about home testing and diet. There's a lot to learn, and it's overwhelming in the beginning, but you can do it! We all have or have had diabetic cats here, and there's a lot of knowledge and experience on this site. Keep reading, and start a new thread when you have questions so others can welcome you properly and answer your questions. Be sure to let us know what the vet says on Monday.
Misschievious, I'm glad to see you say that you want to start home testing. We believe home testing is very important here, and we will help you learn how to do it. Walmart sells Relion meters (their brand) and they are a good choice....easy to use, reliable and the test strips are inexpensive. There are other good choices, the most important thing when choosing a meter is to look at the cost of the test strips...that is your biggest expense for testing, so pick a meter with inexpensive test strips.
For diet, look at Janet & Binky's food list, and pick foods that are low in carbs....below 10%, and I try to stay below 5%....the lower in carbs, the better. The Hill's w/d is a prescription food, it's expensive and dry food is not good for a diabetic cat, or any other cat for that matter! You can buy commercial canned food that is much better for both of your cats, and it doesn't have to be expensive. I feed Special Kitty, it's Walmart's brand and some of the varieties are on the low carb list...it's very inexpensive. Friskies has some low carb, Fancy Feast is a popular food here....you don't need to buy prescription food. Print out the lists and take them with you to the store.
Please don't change your cat's diet until you are home testing. This is important, because when you change to a low carb diet, your cat's blood glucose will drop, and you may need to decrease the insulin dose. If you aren't testing, you could end up in a hypo situation.